Unions like the Los Angeles County Federation of Labor, which is backing Greuel, or the Teamsters, who endorsed rival Eric Garcetti, are making a big push in the Los Angeles mayor's race, sending workers to canvass, phone bank and gather signatures.

But mostly, they are raising money.

Labor groups have contributed significantly to the $10.2 million spent by outside groups in the mayor's race. One union for the Department of Water and Power has broken spending records.

In a race with two pro-labor candidates, with similar backgrounds, the unions' power is noticeable, say political observers.

"The outside spending is changing the campaign narrative," said Jessica Levinson, a professor at Loyola Law School. "It's changing the tenor and tone."

The emergence of SuperPACs, or independent committees, have allowed outside groups, which include unions, to skirt city rules limiting individual contributions to $1,300 per election cycle. Independent committees can raise unlimited funds, but can't coordinate with campaigns.

SuperPACs have spent $7.5 million on Greuel's behalf in the race, more than three times the amount spent by outside groups on Garcetti.

The latest filings show outside groups are now outpacing candidates' fundraising efforts. Greuel has raised $7 million to date, according to the city's Ethics Commission. Even as she runs short of money - the last filings show Greuel has $295,000 cash on hand, limiting her ability to run TV spots - labor groups have filled the gap.

Some of the most powerful unions at City Hall - those representing police, fire, and the Department of Water and Power - have endorsed Greuel and aired advertisements for her.

The County Fed, which represents 300 different unions, is also backing the controller. Unite Here Local 11, which represents hotel workers and is part of the County Fed, endorsed Greuel after she committed to a $15 wage for most hotel workers.

She voiced support for the wage in December, her campaign said, and again at an exclusive endorsement meeting with union leaders in March.

Rival Garcetti angered the hotel union after he sided with a trade group rather than Unite Here on a Hollywood hotel deal.

"We always knew we were going to endorse Greuel," said Unite Here spokeswoman Leigh Shelton, who said Garcetti was never invited to interview for the endorsement.

Like Greuel, Garcetti is receiving support from a mix of city and private unions. Labor groups representing grocery store workers, janitors, and port workers are siding with the councilman. Some of those workers are hoping Garcetti sides with them on City Hall issues such as banning box stores like Wal-Mart or protecting jobs at the L.A. Port.

A union representing longshoremen has backed Garcetti with at least $300,000, while a local Teamsters unions has contributed $100,000.

Teamsters political director Ed Rendon said Garcetti has a better record than Greuel of siding with labor on issues important to his union, like waste recycling.

Despite that backing, the councilman hasn't seen the same financial boost as Greuel. Garcetti has received a little over $2 million in outside funding.

By contrast, Working Californians to Elect Wendy Greuel, an outside group, has backed Greuel with more than $4 million, according to filings with the city Ethics Commission. The International Brotherhood of Electrical Workers Local 18, a DWP union, is the single biggest contributor to that committee.

Additionally, groups led by a union for the city's police officers have raised more than $1.4 million for Greuel.

"The (union) money has been almost all going to Greuel," said Jaime Regalado, professor emeritus of political science at Cal State L.A. "But it doesn't mean that union households are going to go to Wendy. Sometimes the union leadership, they're in the minority."

Unions aren't the only ones writing big checks to the SuperPACs. Former Dodgers owner Frank McCourt has given at least $150,000 to groups backing Greuel. Late-night TV host Jimmy Kimmel, whose Hollywood studio is in Garcetti's council district, has contributed at least $30,000 to the councilman. And Millennium Partners, which wants approval for a skyscraper project, contributed $10,000 to a pro-Garcetti PAC last week.

The flood of money concerns Levinson, who said the outside spending dilutes the "one vote, one voice" concept of the electoral process. The spending also shapes the conversation along the campaign trail, she believes.

A confusing County Fed-backed mailer promising a $15 wage if Greuel is elected forced both candidates to weigh in last week. And Greuel has spent months seeking to prove to voters that she'll be independent from the DWP, despite the union's spending.

At a recent debate in Sherman Oaks, the moderator intervened, asking the candidates to stop bickering about DWP, and move on to new topics.